1H and 13C NMR studies of isomerism in hydroxamic acids

1H and 13C NMR studies of N‐methyl‐substituted hydroxamic acids, RCON(CH3)OH (R = CH3, C2H5 and C6H5), show that the series exhibits cis–trans isomerism about the CN bond. The Z/E ratio increases in the series CH3 < C2H5 < n‐C5H11 < n‐C6H13 for a given solvent, indicating that steric interaction between R and the OH group is more important than that between R and the CH3 group. In DMSO‐d6 the Z isomer is preferentially stabilized by solvation, whereas in C6D6 and toluene‐d8 the unsolvated E form is thermodynamically more stable, as predicted by ab initio molecular orbital calculations. (12) At high concentrations (in CDCl3, CD2Cl2, C6D6 and toluene‐d8) the E isomer may also be stabilized by intermolecular association. In the O‐methyl‐substituted series, RCONHOCH3 (R = CH3, C6H5), although the Z isomer predominates in all solvents, at low temperatures there is evidence for enolic forms.